Multi-panel resize support

[bvaughn]

Resolves #268

• [x] Implement multi-panel support
• [x] Add new unit/e2e tests
• [x] Figure out what to name the "gutter" prop
• [x] Update documentation and CHANGELOG

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Name	Status	Preview	Comments	Updated (UTC)
react-resizable-panels	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 30, 2024 2:59pm